1. Field of the Invention
This invention relates to a tone generator for applying bursts of tone to a telephone line to operate metering equipment at the user's site
2. Description of the Prior Art
In telephone or like systems concerned with public telephones of the type in which the number of calls completed by a subscriber or the number of unit values represented by such calls is recorded on some type of register device at the office as the basis for rendering accounts. It is often desirable in such systems to have the subscriber aware of such recording as it progresses. This feaure is especially desirable in hotels, where the user must render his accounting prior to leaving the premises.
Such systems are often referred to as Periodic Pulse Metering systems. In such an arrangement, where the subscriber is to be informed, an originating junctor sends out a 120 millisecond pulse of 16 KHz to the calling telephone in order to operate a recording device in the telephone. The rate at which these tone pulses are transmitted depends on the toll charges. Thus the subscribers are aware of these charges on their own premises. Also coin telephones are arranged to collect coins in amounts representative of the call units in response to the received 16 KHz tone signals.
However when the 16 KHz signal is applied to the line, an audible click is heard in the calling telephone; when the tone is removed, another click is heard. This appears baffling since the telephone has an upper frequency response limit of only 4 or 5 KHz.
An explanation for this effect is offered in "Radiotron Designer's Handbook", 4th Edition, page 300, in relation to vacuum tubes -- "An even function wave is produced by the distortion introduced by valves."
This function is: EQU F (.theta.) = Bo/2 + B1 cos .theta. = B2 cos 2.theta. + B3 cos 3.theta.+
where Bo/2 is a constant which is zero if the wave is balanced about the x axis. Its value is the average value of the y component over one cycle. Therefore, even if the total harmonic distortion is very small, the Bo/2 component still exists. This direct current component attracts the diaphragm in the telephone receiver, causing the first click. When the 16 KHz is removed, the diaphragm snaps back to its reset position, causing the second click. The loudness of the click depends on the amplitude of the 16 KHz signal.
This action, the presence of a DC component from a sine wave, can be verified experimentally for example by connecting a type 412A Hewlett Packard DC Vacuum Tube voltmeter to the output of a Hewlett Packard 200AB test oscillator via a 0.0047 mfd capacitor. When the oscillator is set to 16 KHz and 2.5 VRMS the DC millivoltmeter reads 0.05 mV. At 5.0 VRMS, the meter reads 0.18 mV; at 10 VRMS, the meter reads 0.65mV. This shows that there is a DC component in the output of the test oscillator even though a DC blocking capacitor is used. The presence of this DC component is not a new phenomenon, it existed in prior systems when dial tone, ringback tone, busy tone, TCMF tones and others were switched on and off. But in these prior applications it was masked or overwhelmed by the sound produced by the telephone receiver. In the 16 KHz application the telephone receiver does not produce a 16 KHz sound so that the clicks are heard distinctly. Further, the trend is to longer lines in the telephone systems with a higher resistance ncessitating the use of higher voltages for the tone thus increasing the volume of the click on the shorter lines.